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A molecule is a set or group of atoms which are chemically bonded. It can be represented by a molecular formula. A molecular formula represents the different kinds of atoms that are present in a molecule, along with the actual ratio of its atomic combination in forming that molecule.


A molecule of H2O (water) contains two hydrogen atoms bonded to one atom of oxygen.

NaOH     Sodium Hydroxide

A molecule of sodium hydroxide contains one sodium atom, one oxygen atom, and one hydrogen atom. The point is that molecular formula represents the molecules and the actual ratio of the atoms present in them.

Molecular Weight

Molecular weight represents the sum of the atomic weights of all the atoms in that molecule. Molecular weight is also known as formula weight.
Example 1-1
Calculate the molecular weight of sulfuric acid (H2SO4).
2 hydrogens      2 x 1 = 2
1 sulfur     1 x 32 = 32
4 oxygens 4 x 16 = 64
98 g/mol
Example 1-2
Calculate the molecular weight of carbon dioxide (CO2).
1 carbon   1 x 12 = 12
2 oxygens  2 x 16 = 32
44 g/mol

Empirical Formula

Empirical formula of a molecule represents the simplest ratio of the atoms present in the molecule.
For example, acetylene has the molecular formula C2H2. The empirical formula of acetylene is CH. In essence, empirical formula gives the simplest ratio of atoms in a molecule.

Problem 1-1
Write the empirical formula of the following molecules.
1. H2O2
2. C2H6
3. H2O
1) HO
2) CH3
3) H2O
Notice that sometimes the empirical formula is the same as the molecular formula as in the case of water (H2O).

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